Neurophysiol 94: 2457-2471, 2005

Neuronal Activity in Primate Orbitofrontal Cortex Reflects the Value of Time
 
Matthew R. Roesch^1,2 and Carl R. Olson<sup>1,2

1</sup>Center for the Neural Basis of Cognition, Mellon Institute; and ²Department of Neuroscience, University of Pittsburgh, Pittsburgh, Pennsylvania

Submitted 12 April 2005; accepted in final form 11 June 2005

Subjects
Two adult male rhesus monkeys were used (Macaca mulatta; laboratory designations P and F). Experimental procedures were approved by the Carnegie Mellon University Animal Care and Use Committee and were in compliance with the guidelines set forth in the United States Public Health Service Guide for the Care and Use of Laboratory Animals.

Preparatory surgery
At the outset of the training period, each monkey underwent sterile surgery under general anesthesia maintained with isofluorane inhalation. The top of the skull was exposed, bone screws were inserted around the perimeter of the exposed area, a continuous cap of rapidly hardening acrylic was laid down so as to cover the skull and embed the heads of the screws, a head-restraint bar was embedded in the cap, and scleral search coils were implanted on the eyes, with the leads directed subcutaneously to plugs on the acrylic cap (Robinson 1963 ). After initial training, recording chambers were implanted into the acrylic. For this purpose, a 2-cm-diameter disk of acrylic and skull overlying the left hemisphere was removed. A cylindrical recording chamber was cemented into the hole with its base flush to the exposed dural membrane. The chamber was centered at approximately anterior 23 mm and lateral 23 mm with respect to the Horsley–Clarke reference frame.

Variable-delay task
The monkeys performed a memory-guided saccade task in which a cue presented early in each trial predicted a short (500-ms) or a long (2,500-ms) delay period. Essential features of the task are summarized in Fig. 1A. Each trial began with onset of a central fixation spot. At a point in time 50 ms after attainment of fixation, the spot was transformed to a cue the shape and color of which signified the length of the upcoming delay period. After 400 ms two potential targets appeared at diametrically opposed locations to the right and left of fixation. A directional cue identical to the fixation cue except in size was then presented for 250 ms in superimposition on one of the targets. After a 500-ms (or 2,500-ms) delay period, the fixation spot was extinguished, whereupon the monkey was required to make a saccade directly to the previously cued target and to maintain fixation on it for 300–450 ms after saccade completion, at which time a juice reward was delivered.